DB's has one Rule of Combat that determines how a Fight would go and i think it's not very realistic. If Fighter A has a much greater Ki level than fighter B, there is no way B can hurt A, even if caught off guard. Wouldn't it have made DB fights more interesting, if this rule didn't apply?

Two words: Original Dragonball. Where skill could turn the tide of a fight.

Also, Goku, Gohan, and Piccolo were all able to hurt Raditz. Goku was able to injure Vegeta, and Frieza despite being much weaker than all of them. Piccolo kicked Frieza and was able to hold him off while Goku finished the Genki Dama. Goku and Vegeta both almost killed Cell. Trunks, despite being stronger than Cell's initial warming up power in USSJ2, was manhandled because he wasn't fast enough. Plenty of people hurt Buu...as much as Buu can be hurt...despite being weaker.

If regeneration hadn't been introduced, Vegeta would have killed both Cell and Buu despite being multiple times weaker than both of them. And prior to that, there were multiple instances of weaker fighters injuring much stronger ones.

As for it being unrealistic, I don't think so. In real life if a young child or teenager hit a body builder - unless it was in the groin or throat or something - the body builder would barely feel a thing. Speaking from experience here.It's not unrealistic, it just seems so because the powers displayed in DBZ are much huger than anything we could ever hope to reach.

Except that Piccolo's Special Beam Cannon was not stronger than Raditz. It's a piercing attack like Frieza's death beams.The Spirit Bomb did not kill Vegeta or even finish him, it just weakened him. He was actually beaten by Gohan.Buu...yeah, okay.

And because these weaker fighters have attacks that have special attributes that allow them to beat stronger fighters, that's not fair? That's HOW weaker fighters win against fighters massively stronger than them.

Raditz was about 3 times stronger than Goku and Piccolo. It doesn't matter how 'smart' they are (thought grabbing his tail TWICE worked). With a gap like that, nothing else would have worked. In real life, if somebody can run three times faster than you can and sustain that pace, it doesn't matter how intelligent you are, you won't beat them in a race. If someone is three times stronger than you, you won't beat them in an armwrestle. Period. And that's why strategy is seldom used in DBZ. Because the gaps were almost always enormous.

Vegeta was 3 times stronger than Goku and about 15 times stronger than Gohan and Krillen.

Because of Buu's regeneration, it would have been utterly impossible for someone weaker than him to kill him unless an attack with massive amplification was used.

There were very few fights in the series where the gap between fighters was close enough to a point where strategy and intelligent fighting would have made a difference:

Piccolo vs 17Goku vs Cell (supressed)Goku vs Vegeta 2

In all three of those fights, there was a lot more strategy involved than just roflstomping the other fighter.

Goku's instant transmission Kamehameha, for example, was brilliant. And again, if Cell did not have regeneration, it would have been over right there.

I would have liked to see something like Mace Windu vs Palpatine or Obiwan vs Anakin, where a fighter combines a style+experience to overpower a much stronger opponent. I suspect Piccolo vs 2nd form Frieza was like that and the filler fight Goku vs Pikkon.

It's hardly even a "rule", it's just common sense that as time passes and powers become greater and greater, that fights would get one-sided. In Dragonball powers were relatively closer to one another, so you'd get more exciting and evenly matched battles. In DBZ and GT, situations like that were extremely rare. They did occur, but just a whole lot less, and with good reason (IMO).